This invention relates generally to micro-electromechanical systems (MEMS). More particularly, this invention relates to the use of MEMS for mixing one or more fluids.
Micro-electromechanical systems (MEMS), which are sometimes called micromechanical devices, micromachines, micro-fabricated devices or nano-structures, are three dimensional objects having one or more dimensions ranging from microns to millimeters in size. The devices are generally fabricated utilizing semiconductor processing techniques, such as lithographic technologies.
The use of MEMS to mix one or more fluids has numerous applications in industries ranging from chemical analysis, to printing, to medicine. As used herein, the term xe2x80x9cmixingxe2x80x9d refers to combining two fluids, increasing the uniformity of a single fluid, decreasing the special or temporal gradients with respect to one or more fluid properties, or increasing small scale decomposed structure from large scale homogenous structure in a fluid.
As previously indicated, there are numerous applications for fluid mixing MEMS. For example, a device capable of mixing, and thereby processing, tens to hundreds of nanoliters of fluid would increase by two orders of magnitude the number of chemical tests that can be performed on a given volume of fluid. In printing, fluid-handling MEMS would allow for the mixing of inks xe2x80x9coff-paperxe2x80x9d, thereby allowing for on-demand ink formation, increasing the print quality and decreasing the amount of ink required. In medicine, fluid-handling MEMS could be implanted under the skin, or incorporated in micro fabricated needles, and programmed to mix and dispense assays according to current need or a pre-programmed schedule. Numerous additional applications exist for fluid-handling MEMS.
The ability to mix fluids thoroughly and in a reasonable amount of time is fundamental to the creation of fully integrated, xe2x80x9con-chipxe2x80x9d MEMS fluid processing systems. Effective mixing of fluids requires that the fluids be manipulated or directed so that the contact area between the fluids is increased. In macroscopic devices this is generally done using turbulence, three-dimensional flow structures, or mechanical actuators. In MEMS, however, it is often difficult and expensive to use such means to effect efficient fluid mixing. MEMS are typically fabricated in a lithographic environment, the design constraints of which mitigate against mechanical actuators. Furthermore, MEMS are essentially planar devices, with the X and Y axes defining the plane of the device. The design of structures in the third-dimensional Z axis (or vertical axis orthogonal to the plane defined by the X and Y axes) is constrained by lithographic techniques. For example, lithographic techniques limit the Z-axis structures to uniform shape and depth throughout the device. As a result, the Z-axis dependence of the flow field will be uniform (e.g., parabolic) throughout the planar device. A flow with uniform Z dependence is referred to as planar flow. It is difficult to achieve mixing in this context.
The size and proportions of MEMS generally preclude relying on either turbulence or diffusion alone as mixing mechanisms. The size of fluid chambers in a MEMS can range from the picoliter, (10 xcexcm)3, to the microliter, mm3, range. Though fabrication constraints allow for picoliter chambers, few commonly used fluids are concentrated enough to be useful in such quantities. An upper bound on volumes of about 50 xcexcl is set by the size of a typical device (10 mmxc3x9710 mmxc3x97500 xcexcm). Process volumes in the 100 nanoliter range allow multiple chambers to be fabricated on one die, yet provide sufficient fluid for many applications.
Turbulence occurs in flows characterized by high Reynolds numbers, defined as
Re=(Uxcex4)/v,xe2x80x83xe2x80x83[1]
where U is a characteristic velocity, xcex4 is a length scale, and v is the kinematic viscosity (1 mm2/s for water). The appropriate length scale, typically the channel height, will in general be smaller than 500 xcexcm. Assuming the highest velocity to be experienced for on-chip flows is one die length per second (U=10 mm/s), an upper bound on the Reynolds number is Re=5, with typical values being much lower. As turbulence in channel flow occurs only for Re greater than 2000, on-chip flows are expected to be laminar, and thus turbulence is not available as a mixing mechanism. Moreover, flows with Re less than  less than 1, known as creeping flows, are symmetric and reversible. In this regime, a flow moving past an object will reconstitute itself, passing by the object unchanged, and xe2x80x9cmixingxe2x80x9d caused by a given set of manipulations to the fluid can be undone simply be reversing the set of manipulations. This precludes the use of barrier-fields, complex geometries, and severely limits the usefulness of mechanical actuators.
Similarly, the size and shape of MEMS limit the usefulness of diffusion as a sole mechanism for mixing. As it is difficult to place two fluids on top of each other in a planar MEMS, the length over which diffusion must act will be the in-plane dimension of the fluid chamber. Using Fick""s equation, a diffusion mixing time scale, TD can be formed
TD=L2/k,xe2x80x83xe2x80x83[2]
where L is the relevant mixing length, and k is the Fickian diffusion constant (k=103 xcexcm2/s for salt in water, for example). Using L=1 mm, TD=103 seconds=16.6 minutes. Even for L=100 xcexcm, TD=10 seconds. Such mixing times are generally too slow to rely on diffusion for effective mixing.
U.S. Pat. No. 6,065,864 (the ""864 patent), which is assigned to the assignee of the present invention and is incorporated by reference herein, discloses seminal work in connection with the use of MEMS for planar laminar mixing. The planar laminar mixing technology described in the ""864 patent is directed toward batch and is generally not as fast and efficient as continuous processing. Accordingly, it would be desirable to provide an improved planar laminar mixing technique to mix fluids in a MEMS.
An embodiment of the present invention is a micro-electromechanical system that includes a first channel for carrying a first stream of fluid, a second channel for carrying a second stream of fluid, and a mixing channel coupled to the first channel and the second channel to receive the streams. The micro-electromechanical device includes mechanisms for generating pulses in the first stream and the second stream such that, when the first stream and the second stream enter the mixing channel, the interface between the two streams becomes distorted. The distorted interface operates to facilitate diffusion between the two streams.
Another embodiment of the present invention is a method of mixing fluids in a micro-electromechanical system. The method includes the steps of: producing pulses in a first stream of fluid, creating pulses in a second stream of fluid, and merging the first stream of fluid and a second stream of fluid in a mixing channel. The pulses, which may be generated according to the dimensions of the mixing channel, distort an interface between the first stream and the second stream to facilitate diffusion between the two streams.
The invention further provides an efficient technique for mixing fluids in a micro-electromechanical system. Mixing is achieved in a two dimensional velocity field without turbulence. The invention facilitates continuous laminar fluid mixing. The pumps of the invention operate to vary the ratio(s) of the mixed fluids and to control the flow rate of the mixture.